Our daughter sucked on the bottom of my wife's iPhone 4 and ruined the speaker. It still played sound but it was muffled and quiet.
Minor panic when I thought it was a 4s and was using the wrong guide. I was worried it was the wrong part I'd ordered but once I got the correct instructions it was smooth sailing.
Make sure you keep track of which screws go where. There are so many at different sizes ... I'm not sure I got all of them back in the correct spots ;-)
There was also 1 flat rubber seal that I didn't see where it came from and couldn't figure out where it goes. It was up near the camera but not 100% sure where.